A tint is generally any hue (color) that has been mixed with white. Tint strength is a measure of how strongly a particular color or pigment affects (or tints) another one when mixed with it. Some pigments (for instance phthalo blue) have a very strong tinting ability, meaning a small quantity will have a great effect on another color or pigment when mixed with it. Others (for instance burnt sienna) have a weak tinting ability, meaning a small quantity will have minimal impact on another color when mixed with it. The tinting ability of paint is generally affected by the size of the pigment particle and how well the pigment particles have been dispersed within paint. The tinting ability of paint is generally improved by grinding a pigment mixture added to the other conventional components of paint, such as binders, solvents and type of conventional grinding devices, such as ball mills used and the duration of grinding. Determining the fineness of paint's grind is important, because a grind, which is too coarse may reduce the paint's color uniformity, gloss, and opacity. The longer the grinding time, the finer would be the pigment particle size. However, if the grinding time is too long, it may not be economically a viable and moreover it could adversely affect the size, i.e., reduce the flake size of some of the paint components, such as metallic flakes generally added to produce a metallic paint to an unacceptable level. Obviously if the grinding time is too short, it would have adverse effect on the tint strength of paint.
Thus, knowing the tint strength of a coating composition while it is being made is very important to a paint maker for producing coating composition of known and uniform tint qualities. Therefore, a paint manufacturer typically would check a sample of a coating composition, such as automotive OEM paint or refinish paint, while it is being made to determine its tint strength. One way to check the tint strength is to use Hegman grind gage, which allows a paint maker to determine how finely ground the particles of pigment (or other solid) are dispersed in a sample of a coating composition by using the procedure described in ASTM D1210. The gage consists of a steel block with a series of very small parallel grooves machined into it. The grooves decrease in depth from one end of the block to the other, according to a scale stamped next to them. A typical Hegman gage is 170 mm by 65 mm by 15 mm, with a channel of grooves running lengthwise, 12.5 mm across and narrowing uniformly in depth from 100 μm to zero. A puddle of a sample of a coating composition is placed at the deep end of the gage and then drawing the coating composition down with a flat edge along the grooves. The paint fills the grooves, and the location where a regular, significant “pepperyness” in the visual appearance of the coating appears, marks the coarsest-ground dispersed particles. The reading is then taken from the scale marked next to the grooves, in dimensionless units called “Hegman units” and/or in mils or micrometers. So from time to time, an aliquot of such coating compositions being manufactured is taken and its tint strength is visually observed with tint measuring devices, such as Hegman grind gage. The process parameters are then adjusted and the aforedescribed testing procedure is repeated until the adjusted coating composition falls within desired tint strength.
However, the aforementioned testing procedure is not only time consuming and cumbersome but it is also not very sensitive to changes to tint strength during the grinding process since the aforementioned procedure involves visual observation, which can change from one observer to the next. As a result, even though the Hegman grind gage may indicate an acceptable level of grinds, the batch-to-batch quality of the resulting coating compositions can be still detrimentally affected due it insensitivity to grinding intervals. Therefore, a need exists to develop a process that could more accurately predict the tint strength of a coating composition while it is still being manufactured such that the manufacturing process could be readily adjusted on a real time basis to get the desired tint strength.